The arteries and arterioles (collectively referred to as “arterial blood vessels”) throughout the body are lined with smooth muscle. This smooth muscle regulates the flow of arterial blood. Relaxation of the smooth muscle dilates the arterial blood vessel, permitting free flow of blood. Contraction of the smooth muscle constricts the arterial blood vessel, thereby constricting the flow of blood.
Relaxation and contraction of smooth muscle also have an effect on blood pressure. Relaxation increases the volume of the arterial blood vessel, thereby reducing blood pressure. Such blood vessels are said to be “vasodilated.” Contraction reduces the volume of the arterial blood vessel, thereby increasing blood pressure. Such blood vessels are said to be “vasoconstricted.”
When a person suffers a heart attack, a tendency exists for blood pressure to fall. In an effort to counteract this tendency, the autonomic nervous system causes smooth muscles throughout much of the vascular system to contract. The resulting system-wide vasoconstriction raises blood pressure.
Among the vasoconstricted arterial blood vessels are the renal artery and its associated renal arterioles. Constriction of these arterial blood vessels hampers the kidneys' ability to remove excess fluid. As a result, blood volume increases beyond normal levels. The heart, which is already in weakened condition, encounters difficulty pumping this quantity of blood. As a result, this excess blood can accumulate in the heart and cause the heart to pump less efficiently. In addition, the excess fluid can be excreted into the lungs, resulting in pulmonary congestion, which in turn may cause difficulty breathing.
It is possible to provide drugs that reduce blood pressure. But these drugs act systemically and therefore counteract what is, for most parts of the arterial system, an effective response to the drop in blood pressure caused by heart failure.
It is also known that smooth muscle tone may be locally controlled by contacting the arterial wall with an electrode and providing a suitable electrical stimulation. However, this requires that the arterial blood vessel be large enough to permit insertion of an electrode. As a result, it is difficult to use this method to control smooth muscle tone in the arterioles. Moreover, there are often a great many arterioles that will need to be vasodilated. A procedure that relies on contacting the wall of each such arteriole would thus be impractical.